They live in the forests and limestone outcrops of Laos. With long whiskers, stubby legs and a long, furry tail, they are rodents unlike any seen before by wildlife scientists. They are definitely not rats or squirrels, and only vaguely like a guinea pig or a chinchilla. And they often show up in Laotian outdoor markets as food.

It was in such markets that visiting scientists came upon the animals, and after long study, determined that they represented a rare find: an entire new family of wildlife. The discovery was announced yesterday by the Wildlife Conservation Society and described in a report in the journal Systematics and Biodiversity.

The new species in this previously unknown family is called kha-nyou (pronounced ga-nyou) by local people. Scientists found that differences in the skull and bone structure and in the animal's DNA revealed it to be a member of a distinct family that diverged from others of the rodent order millions of years ago.

"To find something so distinct in this day and age is just extraordinary," said Dr. Robert Timmins of the Wildlife Conservation Society, one of the discoverers. "For all we know, this could be the last remaining mammal family left to be discovered."

Naturalists had trouble recalling when a new family of mammals was last identified. It may have been in the 1970s, when a new family of bats was found in Thailand. The most active period for finding and classifying new species and families was in the 19th century, when explorers and settlers moved into remote interiors of the continents.

Timmins, who is based in Madison, Wis., but concentrates on research in Southeast Asia, said he first came upon the animals laid out on market tables. Local farmers and hunters trapped or snared the animals, which they also refer to as rock rats, slaughtered them and took them to market. As far as he knew, Timmins said, no Western scientists have ever seen a kha-nyou alive.

Timmins' first encounter occurred in the late 1990s, about the same time that another scientist, Dr. Mark Robinson, independently collected several of the carcasses as specimens. The adults have bodies about a foot long, with a 6-inch tail that is not as bushy as a squirrel's. They knew immediately that this was, as Timmins said, "an oddball rodent."

The specimens were sent for analysis at the Natural History Museum in London by Dr. Paulina Jenkins, a staff zoologist, who compared them with known rodents. Tissue samples were delivered to the University of Vermont for DNA studies by Dr. C. William Kilpatrick, a molecular scientist.

Only in the last year were the two scientists, as well as the two discoverers, ready to publish their findings.

In the journal article, they wrote that the kha-nyou specimens showed "a unique combination of external and craniodental features as members of a new family, genus and species."

The researchers gave the animal the name Laonastes aenigmamus. But they recommended that it be referred to by the common name of kha-nyou or Laotian rock rat. The animals were found in the eastern edge of the Khammouan Limestone National Biodiversity Conservation Area in Laos.

Timmins said he was not tempted to feast on the rock rat and never thought to ask local people what it tasted like. A trip to the market, he said, makes it clear that "in Laos, pretty much everything gets eaten."